The aim of this study is to forecast the number of free-riders in P2P networks which can help
network managers to know the status of the networks in advance and take appropriate measures to cope
with free-riding behavior. Free-riding behavior is common in P2P networks, which has a negative impact
on the robustness, availability and stability of the networks. Severe free-riding behavior may lead to the
crash of the whole P2P application system. Based on the research of free-riding behavior in P2P networks,
this paper constructs a free-rider forecasting model (GST model) using Gray System Theory. Simulation
experiments show that this model has high feasibility, and can carry out reasonable predictions on the
number of free-riders in P2P networks.